This invention relates to a rolling method for steel shapes with parallel flanges such as H-beams and channels which are used in building construction and civil engineering.
Steel shapes having parallel flanges, such as H-beams or parallel-flange channels (herebelow referred to as parallel-flange shapes) have conventionally been manufactured almost entirely by rolling. The name of each portion of a parallel-flange shape will be described while referring to FIGS. 1a and 1b, which are end views of an H-beam and a parallel-flange channel, respectively.
As shown in these figures, both shapes have two parallel flanges 10 which are connected by a web 12 which is integral with the flanges 10. In the case of the H-beam of FIG. 1a, the web 12 is joined to the flanges 10 at their centers, and in the case of the channel of FIG. 1b, the web 12 is connected to one end of each flange 10. The length of a flange 10 to its outer ends is called the flange length Lo, and the distance between the outer edges of two flanges 10 is referred to as the web height Ho. The distance from the inner surface of the web 12 to the end of a flange is called the flange inner length So, and the distance between the inner surfaces of two flanges 10 is called the flange inner width Wo. JIS (Japanese Industrial Standards) includes roughly 33 different standard sizes of H-beams with the web heights Ho ranging from 100-900 mm. Consecutive sizes differ from one another in web height by 25-100 mm.
When an H-beam or a channel is manufactured by a conventional rolling method, as shown in FIG. 2, roughing is performed by a breakdown mill 20. Intermediate rolling is then performed by a universal roughing mill group 26 including a universal roughing mill 22 and a two-high edging mill 24. Finally, finishing is performed by a universal finishing mill 28.
During roughing, a heated rolling material such as an ingot or a continuously cast slab is rolled in the grooves of the breakdown mill 20, which is a two-high reversible mill, to form a beam blank.
Intermediate rolling is performed by mill group 26 consisting of the universal roughing mill 22 and the 2-high edging mill 24 to form an intermediate H-beam. Namely, as shown in FIG. 3, which is a schematic end view of the universal roughing mill 22, the web thickness of the intermediate H-beam 31 is decreased by rolling between the horizontal rolls 30 of the universal roughing mill 22, the flange thickness is decreased by rolling between the lateral surface of the horizontal rolls 30 and the vertical rolls 32, and the rough shape is elongated into an intermediate H-beam 31 by several passes. In each pass of intermediate rolling, the ends of the flanges of the intermediate H-beam 31 are reduced by the grooved rolls 42 of the edging mill 40, and a prescribed flange length Lo is produced. This state is shown in FIG. 4, which is a schematic end view.
During finishing rolling, as shown in FIG. 5, in one or more passes through a universal finishing mill 50, the web 56 and the flanges 58 are reduced in thickness by the horizontal rolls 52 and the vertical rolls 54 in the same manner as in the universal roughing mill 22, the outer surface of the flanges is flattened, and the flanges 58 and the web 56 are made perpendicular to one another.
Thus, when using a conventional rolling method, even in finishing rolling, the inner surfaces of the flanges 58 contact the lateral surfaces of the horizontal rolls 52, and the outer surfaces of the flanges 58 contact the vertical rolls 54, just as in the universal roughing mill 22 used for intermediate rolling. The web thickness is also reduced by the horizontal rolls 52 in the same manner as during intermediate rolling. Accordingly, the flange inner width Wo of a rolled H-beam is determined by the width of the horizontal rolls 52 of the universal finishing mill. This fact causes the following problems.
(1) FIG. 6 shows the cross-sectional shapes of three different H-beams of the same series (H 600.times.200, for example) having the same flange length Lo. Under present standards, for beams in the same series, the flange inner width Wo is constant, so the flange thickness (tfo, tf1, and tf2) is different for beams of different sizes. Furthermore, the web height Ho is different for each beam (Ho, H1, and H2). Namely, tfo&lt;tf1&lt;tf2 and Ho&lt;H1&lt;H2.
The same situation exists with respect to channels of the same series but of different sizes. As shown in FIG. 7, three different channels of the same series have the same inner width Wo, but the web height and the flange thickness of each channel is different.
(2) When rolling shapes having different flange inner widths (Wo), it is of course necessary to change the horizontal rolls of the universal finishing mill. For example, in accordance with JIS, there are 33 different series of H-beams, and in accordance with ASTM, there are 14 different series. In order to manufacture all of these different series of H-beams, at least two horizontal rolls are necessary for each of the 47 different series. At present prices, the cost of all these rolls comes to hundreds of millions of yen. Furthermore, a large and therefore costly building is necessary for the storage of all these rolls, so the investment costs are extremely high.
(3) The horizontal rolls of a single universal finishing mill can roll only 2000 tons/rolling chance .times.3 times =6000 tons of a single series of H-beams. This is because the width of a horizontal roll undergoes about 1 mm of wear per 1000 tons of rolling. Even if the width of a roll is effectively used, the widthwise tolerance of a horizontal roll is only about 6 mm. Therefore, after 6000 tons of rolling, several centimeters are cut off the width of a horizontal roll which can no longer be used for a certain series of beam, and it can then be used for rolling the next series of beam having a smaller web height. Compared to rolls used for rolling steel plate, the amount of steel which can be rolled using a single roll is extremely small. This means that the cost of the rolls per ton of rolled product is high.
(4) When the web height Ho is not a standard height, the normal horizontal rolls on the universal finishing machine must be replaced with special horizontal rolls suited to the web height. Therefore, a small lot of beams having a nonstandard web height cannot be manufactured economically, and manufacturers often refuse orders for small lots of nonstandard beams.
In summary, when a conventional rolling method is used to manufacture parallel-flange shapes, the following problems are encountered.
(1) In a universal finishing mill, it is necessary to have a different set of horizontal rolls with dimensions corresponding to the flange inner width Wo of each series which is to be rolled.
(2) In a single rolling chance, only one series can be rolled.
(3) It is necessary to change the rolls for each series.
(4) A large space is necessary for storage of the rolls.
(5) H-beams having a nonstandard web height H cannot be economically manufactured.
(6) The outer dimensions of the web height Ho are not the same for a single series.
(7) The roll cost is a rather large percentage of the manufacturing costs.
In light of these circumstances, particularly in recent years, built-up H-beams, which are manufactured by cutting a steel plate to form three narrower plates and then welding the three narrower plates together, have become increasingly common and are being used in large quantities. The cost of cutting and welding steel plates makes built-up H-beams more expensive than rolled H-beams, but they do not have many of the above-described disadvantages of rolled H-beams. For example, built-up H-beams can be manufactured in any size, and their dimensional accuracy is superior to that of rolled H-beams.
The above circumstances are generally true of rolled channels as well. However, rolled channels have the following unique problems.
Conventionally, H-beams have been used as the principal structural members of steel-frame buildings. However, as the bending modulus of an H-beam is different depending upon whether the bending force is applied parallel to or normal to the the flanges, H-beams are actually not the most suitable members for use as the main structural members of buildings. Therefore, in recent years, members having a box-shaped cross section have come to be used as main structural members instead of H-beams. Box-shaped members for steel-reinforced buildings of moderate height are commonly electric-resistance welded pipes which have been formed into box shapes. However, for high-rise steel reinforced buildings, box-shaped members are formed by welding large channels together into the shape of a box. The ratio of the flange width Lo to the web height Ho (outer dimensions) of the channels used for this purpose is generally 1:2, so the resulting box-shaped members have a square cross section.
As already mentioned with respect to FIG. 7, in a single series (for example, the 400.times.400 series), there are many different sizes having different flange thicknesses, so even if the flange inner width Wo is constant, the web height Ho is different for each size.
Due to the nature of rolling, it is difficult to remove the bulges 70 which are formed on the outer corners of a channel, so rolled channels are generally used without removing the bulges 70.
In a high-rise steel-reinforced building, the wall thickness of box-shaped members gradually decreases from the bottom to the top of the building. It is common to employ a single series of channels to form the box-shaped members and to gradually decrease the flange thickness from the bottom to the top of the building. However, as the flange thickness decreases, the web height Ho also decreases. Where two channels having a different web height Ho are welded together, there is a step between the two channels along the joint. Furthermore, the size of the bulges 70 on the corners of a channel is different for channels of different sizes, so not only are the bulges 70 unsightly, but they make it difficult to weld two channels together. The same problems occur when channels are used as horizontal girders.
Using a conventional rolling method, if the horizontal rolls of a universal mill are changed for each size of channel, it is possible to maintain the web height Ho constant for a single series of channels. However, as stated above with respect to H-beams, doing so is not economically feasible since it is necessary to have a large number of horizontal rolls and to frequently exchange the rolls.
Japanese Published Unexamined Patent Application No. 61-262403 (1986) discloses a manufacturing method for H-beams which can vary the flange inner width Wo. In that method, after intermediate rolling, the flange inner width Wo is increased using a variable-width rolling mill. Then, during finishing rolling, the flange inner width is finished using segmented rolls. However, an excessive load is applied to the variable-width rolling mill, so that method is difficult to put into practice.